Evaluation of radioiodinated vesamicol analogs forsigma receptor imaging in tumor and radionuclide
receptor therapy
Blackwell Publishing Asia

Kazuma Ogawa,1,2,5 Kazuhiro Shiba,2 Nasima Akhter,3 Mitsuyoshi Yoshimoto,3 Kohshin Washiyama,3 Seigo Kinuya,3Keiichi Kawai3,4 and Hirofumi Mori2
1
4

Graduate School of Natural Science and Technology, 2Advanced Science Research Center, 3Graduate School of Medical Sciences, Kanazawa University, Kanazawa;
Biomedical Imaging Research Center, University of Fukui, Fukui, Japan(Received May 18, 2009/Revised June 30, 2009/Accepted June 30, 2009 /Online publication July 31, 2009)

It has been reported that sigma receptors are highly expressed ina variety of human tumors. In this study, we selected (+)-2-[4-(4iodophenyl)piperidino] cyclohexanol [(+)-pIV] as a sigma receptor
ligand and evaluated the potential of radioiodinated (+)-pIV fortumor imaging and therapy. (+)-[125/131I]pIV was prepared by an
iododestannylation reaction under no-carrier-added conditions withradiochemical purity over 99% after HPLC purification. Biodistribution
experiments were performed by the intravenous injection of(+)-[125I]pIV into mice bearing human prostate tumors (DU-145).
Blocking studies were performed by intravenous injection of (+)[125I]pIV mixed with an excess amount of unlabeled sigma ligand
into DU-145 tumor-bearing mice. For therapeutic study, (+)-[131I]pIV
was injected at a dose of 7.4 MBq followed by measurement of thetumor size. In biodistribution experiments, (+)-[125I]pIV showed highuptake and long residence in the tumor. High tumor to blood andmuscle ratios were achieved because the radioactivity levels of bloodand muscle were low. However, the accumulations of radioactivity
in non-target tissues, such as liver and kidney, were high. The
radioactivity in the non-target tissues slowly decreased over time.
Co-injection of (+)-[125I]pIV with an excess amount of unlabeledsigma ligand resulted in a significant decrease in the tumor/blood
ratio, indicating sigma receptor-mediated tumor uptake. In therapeutic
study, tumor growth in mice treated with (+)-[131I]pIV was significantly
inhibited compared to that of an untreated group. These results
indicate that radioiodinated (+)-pIV has a high potential for sigma
receptor imaging in tumor and radionuclide receptor therapy. (Cancer
Sci 2009; 100: 2188–2192)

I

t has been reported that there are at least two subtypes ofsigma receptors, designated sigma-1 and sigma-2.(1) Whereas
the sigma-1 receptor subtype has been cloned from various
tissues and species,(2,3) the sigma-2 receptor subtype has not yet
been cloned. The functions of sigma receptors have not yet beenclearly defined. In the central nervous system, they have beenshown to be involved in the regulation of neurotransmitter
release, modulation of neurotransmitter receptor function, learningand memory processes, and regulation of movement andposture.(4) Although they are expressed in peripheral tissues suchas liver, kidney, and endocrine organs, their function in thesetissues has been much less understood. At the same time, it has
been reported that both sigma receptor subtypes are highly
expressed in a variety of human tumors such as prostate cancer,
breast cancer, malignant melanoma, glioma, neuroblastoma, andnon-small-cell lung carcinoma.(5,6) The high expression of sigma
receptors in tumors suggests that they are appropriate targets fordeveloping tumor-imaging agents. Furthermore, sigma receptors
should be potential biomarkers of tumor proliferation because
they are highly expressed in rapidly proliferating cells and are
down-regulated when cells become quiescent.(7–9) For the time
Cancer Sci |November 2009 |vol. 100 | no. 11| 2188–2192

Fig. 1. Chemical structure of (+)-2-[4-(4-iodophenyl)piperidino] cyclohexanol [(+)-pIV].

being, radiolabeled sigma ligands should be useful for monitoring
the effects of chemotherapy at an early stage when morphologicchanges are not observed.(10)Previously, we have developed several vesamicol analogs
with iodine into the 4-phenylpiperidine moiety as sigma receptor
imaging agents for investigating the central nervous system,and determined the binding affinities for the sigma receptors ofthe vesamicol analogs.(11,12) In these vesamicol analogs, the (+)enantiomer of 2-[4-(4-iodophenyl)piperidino] cyclohexanol[(+)-pIV] (Fig. 1) showed the highest affinities for the receptors.(12) The inhibition constant (Ki) of (+)-pIV to sigma-1 andsigma-2 was 1.3 nM and 20.4 nM, respectively. The values
mean that (+)-pIV has more than 10-times greater affinity forsigma-1 than that of (+)-pentazocine (Ki = 19.9 nM), which is
known as a sigma-1 ligand, and the sigma-2 affinity of (+)-pIV isequivalent to that of 1,3-di(2-toyl)guanidine (DTG) (Ki = 22.5 nM),
which is known as a non-selective sigma ligand.
In this study, to evaluate the potential of radioiodinated(+)-pIV for tumor imaging and receptor radionuclide therapy,
we selected human DU-145 prostate cancer cells known to overexpress sigma-1 and sigma-2 receptors(13) for preparation oftumor-bearing mice, and a biodistribution study, metabolite
analysis, and therapeutic experiments were performed.
Materials and Methods
Chemicals. [125I]Sodium iodide and [131I]sodium iodide werepurchased from Perkin Elmer (Waltham, MA, USA). TLC
analyses were performed with silica plates (Art 5553; Merck,
Darmstadt,
Germany).
(3,4-Dimethoxyphenethyl)-4-(3phenylpropyl)piperazine (SA4503) was kindly supplied by M’s
Science (Kobe, Japan). Other reagents were of reagent grade
and used as received.
Preparation of (+)-enantiomer of (+)-pIV. The (+)-enantiomer of
p-iodovesamicol [(+)-pIV] was prepared using a method described
previously.(14,15) Briefly, (+)-pIV was synthesized from the (+)enantiomer of vesamicol, which was provided from racemic
vesamicol by recrystallizing the diastereoisomeric salts using

5

To whom correspondence should be addressed. E-mail: kogawa@p.kanazawa-u.ac.jp

doi: 10.1111/j.1349-7006.2009.01279.x© 2009 Japanese Cancer Association

(–)-di-p-toluoyl-D-tartaric acid monohydrate, via a three-stepreaction of nitration, amination, and iodination.
Preparation of (+)-[ 125I]pIV and (+)-[ 131I]pIV. (+)-[125I]pIV and(+)-[131I]pIV were prepared according to procedures described
preciously with a slight modification.(16) Briefly, [125I]sodium
iodide solution (18.5 MBq/5 μL) or [131I]sodium iodide solution(74 MBq/3 μL) was added to 5 μL of 3 M formic acid. (+)-2[4-(4-Tributylstannyl)phenylpiperidino]cyclohexanol [(+)-ptributylstannylvesamicol] (3 μg), synthesized as described
previously,(17) in ethanol (3 μL) and 5 μL of 3% H2O2 was added
and the reaction mixture was gently shaken. After 20 min ofstanding at room temperature, the reaction mixture was quenchedwith 3 μL of 10 M sodium hydroxide solution and then purified
by reversed phase (RP)-HPLC performed with a Cosmosil
5C18-AR 300 column (4.6 × 150 mm; Nacalai Tesque, Kyoto,
Japan) at a flow rate of 1 mL/min with a mixture of acetonitrile,water, and ethanolamine (75:25:0.05) as a mobile phase.
Biodistribution of (+)-[125I]pIV in tumor-bearing mice. Experimentswith animals were conducted in accordance with the Guidelines
for the Care and Use of Laboratory Animals of Kanazawa
University. The animals were housed with free access to foodand water at 23°C with a 12-h alternating light/dark schedule.
DU-145 cells were obtained from ATCC (Manassas, VA, USA)and grown in cell culture dishes in RPMI-1640 medium withphenol red, 10% heat-inactivated fetal calf serum, 100 μg/mL
glutamine, 100 units/mL penicillin, and 100 μg/mL streptomycin.
The cells were cultured in a humidified atmosphere of 95% air
and 5% carbon dioxide at 37°C. They were then released fromthe dishes by treatment with 0.05% trypsin/EDTA. Next, themice to be inoculated were anesthetized with pentobarbital. To
produce tumors, approximately 5 × 106 of the prepared cells
were injected subcutaneously into the right dorsum of 4-weekold-male BALB/c nude mice (15–19 g; Japan SLC, Hamamatsu,
Japan). Biodistribution experiments were performed at approximately 14 –21 days postinoculation, i.e. when tumors reached apalpable size. Groups of four or five mice were intravenously
administered 100 μL of (+)-[125I]pIV (37 kBq). At 1, 24, and48 h postinjection, the mice were sacrificed. Tissues of interest
were removed and weighed, and radioactivity counts were
determined with an auto well gamma counter (ARC-380; Aloka,
Tokyo, Japan) and corrected for background radiation.
Blocking studies. For blocking studies, the above-mentioned
DU-145 tumor-bearing mice were intravenously administered
100 μL of (+)-[125I]pIV (37 kBq) mixed with an excess of each
unlabeled sigma ligand, haloperidol (10 μmol/kg), SA4503(10 μmol/kg), or (+)-pIV (10 μmol/kg), respectively. At 1 hpostinjection, the mice were sacrificed and biodistribution
experiments were conducted as described above.
Metabolite analysis in blood, tumor, and other tissues. For metabolite analysis, the above-mentioned DU-145 tumor-bearing
mice were intravenously administered 100 μL of (+)-[125I]pIV(370 kBq). At 1 and 24 h postinjection, the mice were sacrificed.
Blood was collected by heart puncture using a heparinizedsyringe, and tissues of interest were removed. The blood wascentrifuged at 7000 g for 10 min at 4°C. After collecting theplasma, an equivalent volume of acetonitrile-water (1:1) was
added to the plasma. The mixture was centrifuged at 7000 g for
10 min at 4°C. The tissues of interest (0.2– 0.5 g) were homogenized in 1 mL of acetonitrile-water (1:1). Each homogenizedsample was centrifuged at 7000 g for 10 min at 4°C. The
supernatants were analyzed by TLC with ethyl acetate, hexane,
and ethanolamine (3:7:0.1) as a developing solvent.
Determination of the partition coefficient. The partition coefficient of (+)-[125I]pIV was measured as described previouslywith a slight modification.(18,19) Namely, (+)-[125I]pIV was mixed
with 3 mL each of 1-octanol and phosphate buffer (0.02 M,
pH 7.4) in a test tube. The mixture was vortexed for 10 min.
After vortex, the mixture was centrifuged at 1000 g for 5 min.
Ogawa et al.

2.5 mL of 1-octanol was removed and added to 2.5 mL of newphosphate buffer (0.02 M, pH 7.4). After repeating the sameprocedure twice, 100 μL and 1 mL of 1-octanol and phosphatebuffer were taken and their radioactivity and weight were
measured. The partition coefficient was determined by calculating
the ratio of cpm/mL in 1-octanol to that in the buffer, andexpressed as a common logarithm (log P).
Therapy. The mice were randomly distributed in the experimental groups. When the tumors had reached a palpable size,(+)-[131I]pIV was injected intravenously at a dose of 7.4 MBq. A
group of untreated mice served as a control group. Afterinjection, tumor size was measured with a slide caliper in two
dimensions, and mouse weights were measured 2–3 times per
week. Individual tumor volumes (V) were calculated by theformula V = [length × (width)2]/2 and compared to the values on
the day of treatment (relative tumor volume).
Statistical evaluation. An unpaired Student’s t-test was used
for the therapeutic experiments. One-way anova followed byDunnett’s post-hoc test compared to the control group was used
for experiments in the blocking study. Results were considered
statistically significant at P < 0.05.
Results
Preparation of (+)-[125I]pIV. (+)-[125I]pIV and (+)-[131I]pIV wereprepared by the iododestannylation reaction under no-carrieradded conditions with high radiochemical yield (87% and 83%,
respectively). After purification by RP-HPLC, (+)-[125I]pIV and(+)-[131I]pIV showed radiochemical purities of over 99%.
Biodistribution experiments of (+)-[125I]pIV in tumor-bearing
mice. Table 1 lists the biodistribution of (+)-[125I]pIV in DU-145

tumor-bearing mice. (+)-[125I]pIV showed high uptake and longretention in the tumors. Since the radioactivity levels in bloodand muscle were low, high tumor to blood and muscle ratios
were achieved. However, the accumulation of radioactivity inabdominal organs such as liver and kidney was high. The
clearance of radioactivity in these organs was faster than that in
the tumor. At the same time, the accumulation of (+)-[125I]pIV
in the stomach was low, indicating that deiodination was notobserved in vivo.
Blocking studies. The effects of some sigma ligands on tumor
uptake of (+)-[125I]pIV at 1 h postinjection are shown as %injected dose per gram in tumor in Figure 2. Co-injection ofan excess amount of haloperidol, SA4503, or (+)-pIV resulted
in a significant decrease in tumor uptake after injection of(+)-[125I]pIV.

Fig. 2. Comparison of tumor uptake (mean ± SD) of (+)-[125I]pIV at 1 hpostinjection under no-carrier-added conditions and under co-injection of
haloperidol, SA4503, or (+)-2-[4-(4-iodophenyl)piperidino] cyclohexanol[(+)-pIV]. Significance was determined using one-way ANOVA followed byDunnett’s post-hoc test (*P < 0.05, **P < 0.01 vs. control).

Cancer Sci |November 2009 | vol. 100 | no. 11 | 2189
© 2009 Japanese Cancer Association

Table 1. Biodistribution of radioactivity after intravenous injection of(+)-[125I]pIV in tumor-bearing miceTime after injection
Tissue
Blood
Tumor
Liver
KidneyIntestine
SpleenLung
Heart
Stomach†Brain
MuscleT/B ratio
T/M ratio

1h

24 h

48 h

0.24
(0.03)
6.27(1.00)
16.91(0.79)
16.24(2.52)
6.44(1.10)
13.62(2.73)
15.62(5.87)
8.21(1.85)
0.66(0.09)
7.65(2.15)
2.41(0.77)
26.20(4.23)
2.72(0.55)

0.71(0.05)
8.78(0.41)
15.23(0.62)
9.38(0.60)
4.89(0.16)
4.04(0.53)
3.83(0.44)
1.62(0.21)
0.59(0.05)
2.89(0.10)
0.64(0.17)
12.39(0.98)
14.30(2.76)

0.37(0.06)
6.04(1.19)
5.24(0.41)
4.21(0.28)
1.50(0.19)
1.75(0.25)
1.81(0.62)
0.65(0.07)
0.27(0.05)
1.08(0.09)
0.30(0.09)
16.48(2.16)
20.54(2.02)

Fig. 3. Curves depicting inhibition of growth of DU-145 on therapy with(+)-[131I]pIV (closed circles) compared with no treatment (open circles).
Data are expressed as tumor volume relative to that on the day oftreatment (mean ± SEM for four mice). Significance was determinedusing the Student’s t-test (*P < 0.05, **P < 0.01 vs. untreated group).

Data are expressed as % injected dose per gram tissue. Each value
represents the mean (SD) for four or five animals.
†
Data are expressed as % injected dose.
T/B ratio, tumor : blood ratio; T/M ratio, tumor : muscle ratio.

Table 2. Analysis of metabolites after intravenous injection of (+)[125I]pIV in tumor-bearing miceTime after injection
Tissue
Blood
Tumor
Liver
Kidney
Lung
Brain

1h

24 h

3.7
(2.6)
83.1
(6.0)
76.0
(5.3)
86.5
(5.5)
86.2
(4.0)
94.8
(2.6)

1.4
(1.5)
40.7
(2.8)
10.4
(2.2)
8.7
(2.0)
10.8
(0.8)
22.8
(1.3)

Data are expressed as % of intact (+)-[125I]pIV. Each value represents themean (SD) for three samples.

Metabolite analysis in blood, tumor, and other tissues. Table 2
lists the results of the metabolite analyses after intravenous
injection of (+)-[125I]pIV in DU-145 tumor-bearing mice. At 1 hpostinjection, large proportions of radioactivity existed as anintact form in almost all tissues except blood. In contrast, at 24 h
postinjection, the proportions of the intact form in all tissues
were much lower than those at 1 h postinjection. At 24 h

2190

Fig. 4. Body weight of DU-145 tumor-bearing mice treated with (+)[131I]pIV (closed circles) or with no treatment (open circles). Data areexpressed as relative value to initial body weight (mean ± SEM for fourmice).

postinjection, radioactivity in the tumor showed a relatively
large proportion of the intact form (40.7%) compared withthose in other tissues. In blood, almost no intact (+)-[125I]pIV
was observed at 1 and 24 h postinjection (3.7% and 1.4%,
respectively).
Determination of the partition coefficient. The log P-value of(+)-[125I]pIV was 2.08 ± 0.02 (mean ± SD for four samples).
Therapy. The volume of the tumors as a function of time isshown in Figure 3. As can be seen, tumor growth in the micetreated with (+)-[131I]pIV was significantly inhibited comparedto that of the untreated group. Weight could be used as a marker
for the general health of the mice (Fig. 4). The mice treated with(+)-[131I]pIV lost weight, which was probably caused bytreatment-related toxicity, but the weight loss was less than 10%of their starting body weight.
doi: 10.1111/j.1349-7006.2009.01279.x© 2009 Japanese Cancer Association

Discussion

In the case of the differential diagnosis of tumors, information
obtained from Single photon emission computed tomography(SPECT) or PET imaging should be more important than thatfrom other imaging modalities such as CT, MRI, or ultrasound.
That is to say, among those imaging modalities, nuclear medicine
modality is most advantageous for detecting functional changes
such as changes of receptor density, metabolism, and so on. It
has been reported that the density of sigma receptors affects thecondition of tumors.(7,20) Therefore, nuclear imaging agents forsigma receptors could be useful in the differential diagnosis oftumors.(9) In this study, we first examined the potential ofradioiodine-labeled (+)-pIV, which has a very high affinity forsigma receptors, as a tumor-imaging agent.
In the biodistribution experiments in the tumor-bearing mice,(+)-[125I]pIV showed a high uptake of radioactivity in the DU-145
tumor as we expected. Tu et al. described that not only receptor
affinity but also optimal lipophilicity is important in the designof receptor-based tumor-imaging agents.(21) The measured
partition coefficient (log P) for (+)-[125I]pIV was 2.08. The value
was much less than what we expected because the calculated log
P-value for pIV was 4.45 using CS ChemDraw Ultra software(Cambridge Soft, Cambridge, MA, USA). A slight contaminationor the presence of hydrophilic impurities could underestimate
the log P-value. Then, it might be difficult to accurately determine
the log P-value for a high lipophilicity radioactive compound
via the shake-flask method. Although it is not clear whether the
lipophilicity of (+)-pIV is optimal for tumor uptake, part of thehigh uptake in tumors might be derived from the appropriate
lipophilicity of (+)-[125I]pIV. Additionally, since (+)-[125I]pIV
showed low radioactivity in blood and muscle, high tumor toblood and muscle ratios were achieved (Table 1). In order todetermine in vivo binding of (+)-[125I]pIV to sigma receptors,(+)-[125I]pIV was co-injected with excess amounts of some typesof sigma ligands. The co-injection significantly decreased tumor
uptake of radioactivity (Fig. 2), which indicates the receptor
specificity of (+)-[125I]pIV uptake in DU-145 tumors in vivo.
These results indicate that (+)-pIV labeled with I-123 and I-124
instead of I-125 as a radionuclide could be useful as sigma
receptor-imaging agents in tumors for SPECT and PET, respectively. Meanwhile, from our previous in vitro experiments,(12) we
assume that (+)-[125I]pIV prefers sigma-1, but could bind to not
only sigma-1 but also sigma-2. Sigma-1 and sigma-2 receptors
are highly expressed in DU-145 cells.(13) In this study, SA4503
inhibited tumor uptake of (+)-[125I]pIV more strongly than with
haloperidol. SA4503 binds mainly to sigma-1, and haloperidol
is a non-selective sigma ligand.(22) Therefore, the results fromthe in vivo experiments in this study suggest that (+)-[125I]pIVbinds to sigma-1 receptors, but it is not clear whether (+)-[125I]pIV
also binds to sigma-2 receptors.
The problem of the tracer was high accumulations of radioactivity in non-target tissues, such as liver and kidney. As
described in the introduction section, since sigma receptors are
expressed in not only tumor but also non-target tissues such as
in the liver and kidney, some parts of the accumulation of theradioactivity in non-target tissues should be sigma receptor–
specific. It may be difficult to avoid some uptakes in non-target
tissues for labeled sigma ligands. Tu et al. mentioned thatsigma-1-selective or sigma-1/sigma-2-non-selective compounds
might not be ideal candidates for imaging tumors because manykinds of tumors possess a higher density of sigma-2 receptors
than the surrounding normal tissue. On the other hand, the density

References
1 Quirion R, Bowen WD, Itzhak Y et al. A proposal for the classification of
sigma binding sites. Trends Pharmacol Sci 1992; 13: 85–6.
Ogawa et al.

of sigma-1 receptors in tumor cells is generally less than that
present in normal tissues.(21) Although it was reported that rat
liver and kidney contain high densities of sigma-1 and sigma-2
receptors with sigma-2 sites comprising 75–80% of the total
sigma receptors population,(23) Mach et al. performed biodistribution experiments with a 18F-labeled compound possessing
a high affinity for both sigma-1 and sigma-2 receptors withco-injection of the unlabeled sigma-1 selective ligand and showedthat the blocking of sigma-1 receptors reduced the accumulation of radioactivity in non-target tissues except liver.(24) Theirstudies indicate that labeled specific sigma-2 ligands could
show less accumulation in non-target tissues compared withsigma-1-specific or non-selective ligands. At the same time,
lipophilicity might affect not only tumor uptake but also
non-target tissue uptake and higher lipophilicity might lead tohigh non-specific uptake in both target and non-target tissues.(25,26)
Some parts of the accumulation of (+)-[125I]pIV in non-target
tissues, especially in liver, should be derived from its lipophilicity.
Although the radioactivity in the non-target tissues gradually
decreased compared to that in tumor, the clearance of theradioactivity in the non-target tissues was not fast. Consequently, it most likely interferes with the acquisition of clear
images at early times after injection of radioiodine-labeled(+)-pIV.
In the metabolite studies, the clearance of the intact (+)[125I]pIV in tumors was slower than that in any other organ(Table 2). When we calculated (% injected dose per gram)× (ratio of intact (+)-[125I]pIV) at 24 h postinjection of (+)[125I]pIV, the values for blood, tumor, liver, kidney, lung, andbrain were 0.01, 3.57, 1.58, 0.82, 0.42, and 0.66, respectively.
Much more intact (+)-[125I]pIV remained in tumors comparedwith that in other organs. Whether these results were caused bydifferences of metabolic rate is not clear from our data. Even if
more intact (+)-[125I]pIV in tumors is caused by a slow metabolic rate, further studies would be needed to elucidate thecauses. However, more intact (+)-[125I]pIV should affect themore prolonged residence in tumors than that in other organs.
Actually, previous studies have reported that the clearance of thesigma ligands from the brain was faster than those fromtumors.(13,24,27,28) The long residence of radiotracer in tumors isconvenient for internal radiotherapy. Accordingly, we supposedthat radioiodinated (+)-pIV could also be used as an internal
radionuclide therapy by using I-131, which emits middle energyof beta particles for therapy, instead of I-125. We examined thepotential of (+)-[131I]pIV as a therapeutic agent. As a result, a
single treatment with (+)-[131I]pIV (7.4 MBq/mice) achieved
significant inhibition of tumor growth in tumor-bearing micecompared with a control group (Fig. 3). As far as we know, thiswas the first trial to use a radiolabeled sigma ligand for receptor
radionuclide therapy. Although the data are insufficient, theseresults indicate that (+)-[131I]pIV has potential as an agent forradionuclide receptor therapy.
In conclusion, radioiodinated (+)-pIV has a high potential
for sigma receptor imaging in tumor and radionuclide receptor
therapy because of its high tumor uptake via sigma receptor inan animal model. However, further studies are needed in orderto reduce the radioactivity levels in non-target tissues.
Acknowledgment
This work was supported in part by Grants-in-Aid for Scientific Researchfrom the Ministry of Education, Culture, Sports, Science and Technologyof Japan.

2 Hanner M, Moebius FF, Flandorfer A et al. Purification, molecular cloning,and expression of the mammalian sigma1-binding site. Proc Natl Acad Sci
USA 1996; 93: 8072 –7.
3 Kekuda R, Prasad PD, Fei YJ, Leibach FH, Ganapathy V. Cloning andCancer Sci |November 2009 | vol. 100 | no. 11 | 2191
© 2009 Japanese Cancer Association

functional expression of the human type 1 sigma receptor (hSigmaR1).
Biochem Biophys Res Commun 1996; 229: 553–8.
4 Su TP. Delineating biochemical and functional properties of sigma receptors:
emerging concepts. Crit Rev Neurobiol 1993; 7: 187–203.
5 Vilner BJ, John CS, Bowen WD. Sigma-1 and sigma-2 receptors are
expressed in a wide variety of human and rodent tumor cell lines. Cancer
Res 1995; 55: 408–13.
6 Bem WT, Thomas GE, Mamone JY et al. Overexpression of sigma receptors
in nonneural human tumors. Cancer Res 1991; 51: 6558–62.
7 Mach RH, Smith CR, al-Nabulsi I, Whirrett BR, Childers SR, Wheeler KT.
Sigma 2 receptors as potential biomarkers of proliferation in breast cancer.
Cancer Res 1997; 57: 156–61.
8 Wheeler KT, Wang LM, Wallen CA et al. Sigma-2 receptors as a biomarker
of proliferation in solid tumours. Br J Cancer 2000; 82: 1223–32.
9 Hirata M, Mori T, Umeda T, Abe T, Yamamoto T, Ohmomo Y. Evaluationof radioiodinated 1-[2-(3,4-Dimethoxyphenyl) ethyl]-4-(2-iodophenylpropyl)
piperazine as a tumor diagnostic agent with functional sigma receptor
imaging by single photon emission computed tomography. Biol Pharm Bull
2008; 31: 879– 83.
10 van Waarde A, Shiba K, de Jong JR, Ishiwata K, Dierckx RA, Elsinga PH.
Rapid reduction of sigma1-receptor binding and 18F-FDG uptake in rat
gliomas after in vivo treatment with doxorubicin. J Nucl Med 2007; 48:
1320–6.
11 Shiba K, Yano T, Sato W, Mori H, Tonami N. Characterization ofradioiodinated (-)-ortho-iodovesamicol binding in rat brain preparations. LifeSci 2002; 71: 1591– 8.
12 Shiba K, Ogawa K, Mori H. In vitro characterization of radioiodinated (+)2-[4-(4-iodophenyl) piperidino]cyclohexanol [(+)-pIV] as a sigma-1 receptor
ligand. Bioorg Med Chem 2005; 13: 1095–9.
13 John CS, Vilner BJ, Geyer BC, Moody T, Bowen WD. Targeting sigma
receptor-binding benzamides as in vivo diagnostic and therapeutic agents forhuman prostate tumors. Cancer Res 1999; 59: 4578–83.
14 Shiba K, Mori H, Matsuda H et al. Synthesis of radioiodinated analogs of2-(4-phenylpiperidino) cyclohexanol (vesamicol) as vesamicol-like agent.
Nucl Med Biol 1995; 22: 205–10.
15 Rogers GA, Parsons SM, Anderson DC et al. Synthesis, in vitro
acetylcholine-storage-blocking activities, and biological properties ofderivatives and analogues of trans-2-(4-phenylpiperidino) cyclohexanol(vesamicol). J Med Chem 1989; 32: 1217–30.
16 Akhter N, Shiba K, Ogawa K et al. A change of in vivo characteristics

2192

depending on specific activity of radioiodinated (+)-2-[4-(4-iodophenyl)
piperidino]cyclohexanol [(+)-pIV] as a ligand for sigma receptor imaging.
Nucl Med Biol 2008; 35: 29–34.
17 Shiba K, Ogawa K, Ishiwata K, Yajima K, Mori H. Synthesis and binding
affinities of methylvesamicol analogs for the acetylcholine transporter andsigma receptor. Bioorg Med Chem 2006; 14: 2620– 6.
18 Wilson AA, Jin L, Garcia A, DaSilva JN, Houle S. An admonition whenmeasuring the lipophilicity of radiotracers using counting techniques. Appl
Radiat Isot 2001; 54: 203–8.
19 Ogawa K, Mukai T, Arano Y et al. Rhenium-186-monoaminemonoamidedithiolconjugated bisphosphonate derivatives for bone pain palliation. Nucl Med
Biol 2006; 33: 513–20.
20 Aydar E, Onganer P, Perrett R, Djamgoz MB, Palmer CP. The expression
and functional characterization of sigma 1 receptors in breast cancer celllines. Cancer Lett 2006; 242: 245–57.
21 Tu Z, Dence CS, Ponde DE et al. Carbon-11 labeled sigma 2 receptor ligands
for imaging breast cancer. Nucl Med Biol 2005; 32: 423–30.
22 Hashimoto K, Ishiwata K. Sigma receptor ligands: possible application as
therapeutic drugs and as radiopharmaceuticals. Curr Pharm Des 2006; 12:3857–76.
23 Hellewell SB, Bruce A, Feinstein G, Orringer J, Williams W, Bowen WD.
Rat liver and kidney contain high densities of sigma 1 and sigma 2 receptors:
characterization by ligand binding and photoaffinity labeling. Eur J
Pharmacol 1994; 268: 9–18.
24 Mach RH, Huang Y, Buchheimer N et al. [18F]N-4′-fluorobenzyl-4-(3bromophenyl) acetamide for imaging the sigma receptor status of tumors:comparison with [18F]FDG, and [125I]IUDR. Nucl Med Biol 2001; 28: 451–8.
25 Garg S, Kothari K, Thopate SR, Doke AK, Garg PK. Design, synthesis, andpreliminary in vitro and in vivo evaluation of N-(2-diethylaminoethyl)-4[18F]fluorobenzamide ([18F]-DAFBA): a novel potential PET probe to image
melanoma tumors. Bioconjug Chem 2009.
26 Brandau W, Niehoff T, Pulawski P et al. Structure distribution relationshipof iodine-123-iodobenzamides as tracers for the detection of melanotic
melanoma. J Nucl Med 1996; 37: 1865–71.
27 Kawamura K, Kubota K, Kobayashi T et al. Evaluation of [11C]SA5845 and[11C]SA4503 for imaging of sigma receptors in tumors by animal PET. Ann
Nucl Med 2005; 19: 701– 9.
28 Waterhouse RN, Collier TL. In vivo evaluation of [18F]1-(3-fluoropropyl)-4(4-cyanophenoxymethyl) piperidine: a selective sigma-1 receptor radioligand
for PET. Nucl Med Biol 1997; 24: 127–34.

doi: 10.1111/j.1349-7006.2009.01279.x© 2009 Japanese Cancer Association